A fundamental question in neurobiology concerns the neurotransmitter specificity of the neuron. Identified invertebrate neurons offer the advantage of permitting the correlation of this specialization of the neuron with its biochemical properties. The overall objective of this project is the development of a "simple" invertebrate model system in which to study the specific biochemical determinants of neurotransmitter specificity. An immediate objective of this study is to identify and characterize an accessible and experimentally manipulative system involving 5-HT as a transmitter. The presence and the identification of neurons utilizing different neurotransmitters allow comparative biochemical analyses between these neurons. This study will rely upon sensitive and specific radiochemical assays in microbiochemical procedures to measure single cell levels of transmitter system entities in identified neurons. Other biochemical parameters which will be studied include 1) factors influencing the regulation of transmitter biosynthesis, 2) the neuronal packaging and intra-axonal transport of transmitter, 3) factors regulating the release of transmitter, 4) the mechanism of transmitter removal from the synapse, and 5) the interaction of the transmitter with the post-synaptic receptor.